If you click on it a bunch, it will starts to flicker and sometimes create a "dublicate", as in you click on a for example a bow several times and you got two: one in the inventory slot and one in your mouse. Sometimes nothing happens and sometimes the item disappear.
I think it is linked to an other bug I listed "Item disappearing when moved in inventory" MC-4108. Maybe it has something to do with the server, because it's on multiplayer. This ONLY happens in gamemode 1.